The present invention relates to chlorine dioxide production, and more particularly to apparatus for production of chlorine dioxide in a continuous-flow process.
Chlorine dioxide is of considerable commercial importance as a bleaching agent used in paper production, and is becoming increasingly important for use in aqueous solution as an oxidant biocide for water purification, odor removal, slime removal, and treatment of industrial wastes. Production of chlorine dioxide has, in the past, been accomplished through numerous different chemical reactions, and in many different types of apparatus. Previously known processes and apparatus have, however, presented numerous difficulties, and have not been particularly efficient.
A serious problem of production of chlorine dioxide is that chlorine dioxide gas readily forms a dangerously explosive mixture with air. Kesting U.S. Pat. No. 2,664,341 discloses a continuous-flow process and apparatus for the production of chlorine dioxide in a non-explosive manner by a reaction of sodium chloride, sodium chlorate, water and hydrochloric acid, carried out in a number of reaction vessels in a descending arrangement to produce a mixture of chlorine dioxide and chlorine. The Kesting apparatus, however, is pressurized, as it requires addition of steam and air to heat the reactants and to dilute the chlorine dioxide produced, to prevent explosive mixtures of chlorine dioxide and air from being formed. The gases evolved in each stage of the Kesting process are forced through interconnecting tubing into the liquid reactants in the next higher stage, as a means of transferring heat from stage to stage of the reactor apparatus. As a result of this pressurized gas flow, the efficiency of liberation of chlorine dioxide is reduced.
Another problem with production of chlorine dioxide has been that the chemical reactions used in many previously known continuous-flow processes also produce free chlorine gas. For example, the processes disclosed in Rapson U.S. Pat. Nos. 2,863,722 and 3,789,108, Kesting U.S. Pat. No. 2,664,341, Fuller U.S. Pat. No. 3,974,266 and Partridge U.S. Pat. No. 3,563,702 all produce chlorine gas along with chlorine dioxide. While for many bleaching applications solutions containing both chlorine and chlorine dioxide may be useful, free chlorine gas may form compounds which are unwanted in some applications, and, in particular, chlorine is likely to form compounds such as trihalomethanes, which are known carcinogens. Therefore, it is desirable to have a continuous-flow process for production of chlorine dioxide for food related applications, without simultaneous production of free chlorine.
Additional processes for production of chlorine dioxide are disclosed, for example, in Jaszka U.S. Pat. No. 3,950,500, and in Rapson U.S. Pat. No. 2,481,240, which discloses apparatus and a continuous-flow process for production of chlorine dioxide by reaction of sodium chlorate with sulfur dioxide, a poisonous gas, in a packed column reactor.
Partridge et al. U.S. Pat. No. 3,563,702 discloses use of a single vessel generator-evaporater-crystallizer in a process for producing chlorine dioxide. In the Partridge process a venturi eductor is used to reduce pressure within the vessel to remove the gaseous reaction products. Partridge, however, does not further disclose particular apparatus applicable for production of chlorine dioxide.
Another known apparatus for production of chlorine dioxide comprises a packed column into which metered amounts of sodium chlorite and acid are continuously fed at the bottom. Air admitted at the bottom sweeps the reactants and gaseous products of reaction upward through the packed column. An open-ended drain tube connected to a venturi nozzle extends upward to a point near the top of the packed column to maintain the liquid reactant level and remove both liquids and gases, while the venturi reduces pressure within the apparatus. While this apparatus is an improvement over a simple enclosed vessel and a batch process, it is still relatively inefficient in terms of conversion of all available chlorine to chlorine dioxide.
It is well known that sodium hypochlorite and sodium chlorite react with sulfuric acid, yielding chlorine dioxide, sodium chloride, sodium sulfate, and water, in a reversible reaction. In the past this reaction has been carried out in a batch process, as mentioned in Woodward U.S. Pat. No. 2,745,714. While this reaction produces chlorine dioxide without the undesirable chlorine by-product of several continuous-flow processes, the apparatus formerly used has been an inefficient way to produce chlorine dioxide, in terms of the amount of chlorine dioxide produced from a given input of sodium chlorite, and of production in a given amount of time.
What is needed, therefore, is an inexpensive efficient continuous-flow apparatus for producing chlorine dioxide without chlorine gas, and without production of dangerously explosive mixtures of chlorine dioxide and air. It is also desirable to simultaneously produce a solution of chlorine dioxide in water, ready for use in bleaching or as a biocide, and also free from danger of explosion, and to produce the chlorine dioxide without having to handle poisonous gases in the process.